Computer Simulation Study On The Self-assembly Of Tethered Nanoparticles With Tunable Shapes

Tethered nanoparticles?TNPs?^[1]composed of chemically distinct and geometrically anisotropic building blocks provide a promising strategy for bottom-up material design.TNPs are usually composed of"soft chains"and shape-persistent"rigid head"?i.e.,nanoparticle?.These amphiphilic molecules,which have both hydrophilic and hydrophobic groups,are also called giant molecules^[2]-[4].The aggregate structures of giant molecules are determined by the interaction between hydrophobic groups and hydrophilic groups in solution.For the TNP system,various shapes,distinct symmetry,and topology are considered as very important parameters to control their self-assembly process.A wide variety of TNP self-assembly structures have been reported with potential applications in functional materials and nanodevices^[5]-[7].TNPs have shape-and volume-persistent nanoparticles,which bring geometric constraints to the self-assembly process.Hence,the self-assembly of TNPs has unique properties compared with traditional block copolymers.On the one hand,nanoparticles with various shapes such as sphere^[8],rod^[9],and cube^[10],provide many possibilities to study geometrical effects of rigid nanoparticles on their self-assembly.On the other hand,benefited from the booming synthesis techniques,the shape and size of many nanoparticles can change in response to external stimuli and molecular input^[11].Unfortunately,the effect of shape of TNPs and corresponding shape variation on their self-assembly structures has not been systematically studied,even though such an effect is apparently unique for TNPs.For a better understanding of the self-assembly behaviour of TNP in shape-changing process,a suitable model is needed.However,there have been no simulation model and studies available considering shape-changing of TNP.In this study,we have established a TNP model in which the variation of the head of TNP can be taken into account,to implicitly mimic the important influence of ligand chains.We build a TNP model that is composed of a nanoparticle with a tethered polymer chain.The shape of the nanoparticle can be tuned from a pure rigid cube to a soft sphere,mimicking the increasing of grafting density on the nanocube surfaces.With this model,we study the self-assembly of TNPs in dilute solution using dissipative particle dynamics simulation technique,and especially focus on the influence of particle shape,tethered chain length,and grafting density on the self-assembly structures.Some intriguing aggregates such as spherical micelles,pearl-necklace-like structures,cubic columnar structures,handshake structures,core-shell-corona micelles,and four-patch micelles have been observed when varying the interactions between cubes and solvents and the lengths of tethered chain.Modifying nanocube surface with some hydrophilic grafted chains helps the TNPs form small micelles.Increased steric repulsion due to chain overlapping at larger grafting densities results in shape transformation of the nanoparticle from a rigid cube to a soft sphere.In these cases,the self-assembled structures are characterized by the packing of nanoparticles on the micelle surface,and the typical packing mode turns from rectangular?typical for cubes?to hexagonal?typical for spheres?.Our simulation results show new possibilities to design the self-assembly structures of TNPs and illustrate the importance of nanoparticle shapes on determining packing modes and structures of TNPs.